Slanted gradient electron gun



March 14, 1961 E. J. COOK 2,975,324

SLANTED GRADIENT ELECTRON GUN Filed June 24, 1959 2 Sheets-Sheet 1 Edward J. Cook,

u/fimgq His Afforney.

March 14, 1961 Filed June 24, 1959 E. J. COOK 2,975,324

SLANTED GRADIENT ELECTRON GUN 2 Sheets-Sheet 2 llll His A flame y.

SLANTED GRADIENT ELECTRON GUN Edward J. Cook, Burnt Hills, N.Y., assignor to General Electric Company, a corporation of New York Filed June 24, 1959, Ser. No. 822,545

Claims. (Cl. 315'-'-5.31)

This invention relates to an electron gun for producing a hollow beam of electrons useful in an electron tube.

For various electron tube functions it is frequently de sirable and expedient to produce a hollow electron beam which may interact with other tube apparatus, such as slow wave structures, for example, to perform certain functions. Also, it is usually advantageous to maintain the thickness of such a beam at a constant value along its length and avoid spreading of the beam in radial directions.

In certain electron tube structures an interaction region comprising a pair of concentric, coextensive, right circular, cylindrical members, spaced from a cathode, contains a slow wave structure such as a helix with which an electron beam interacts. To obtain maximum effectiveness of beam interaction, it is desirable to produce a thin hollow tubular beam which travels in closely spaced relation to the slow wave structure and it is further desirable to prevent the beam from spreading to any appreciable extent, Different techniques have been devised for pro ducing such a thin beam, many of which include a collimating magnetic field having flux lines directed parallel to the electron path and which permeate the interaction region. in such cases, the magnetic field producing apparatus is large and bulky, requires considerable power for operation and adds to the tube expense. A simplified technique for producing and maintaining such a beam is that in which no collimating magnetic field is required and angular motion is imparted to electrons emitted from a cathode by a radially directed magnetic field adjacent to the face of an annular cathode. The electrons travel in orbital as well as longitudinal paths from the cathode to the longitudinally spaced interaction region. By reason of the orbital motion of the electrons, a centrifugal force acts on them and carries them into progressively larger orbits whereby the entry space into the interaction region is necessarily made at a radius greater than the cathode radius. in the interaction region in such tubes, however, the centrifugal force acting on electrons is balanced by electric field forces acting on the electrons produced by potential differences applied to the cylindrical members between which the electrons travel. Accordingly, after the electrons enter the interaction region, a substantially constant radius, thin beam is maintained. Since, in such prior structures, the kinetic energy of the beam is predominantly in longitudinal motion, the beam interaction with the slow wave structure merely slows the electrons in their longitudinal travel.

In electron guns as hereinabove briefly described and in which orbital motion is imparted to the electrons for the purpose of maintaining tubular flow, the described technique has been eifective for establishing and maintaining a beam thin and tubular. However, in other electron tubes, as for example wherein the kinetic energy of the electron beam is predominantly in the orbital motion of the electrons rather than in the longitudinal motion thereof and in achieving such a beam, a relatively intense radial, magnetic field is required in the cathode region.

2,975,324 Patented Mar. 14, 1951 In producing such a relatively intense field, deviation from an idealized radially directed field is more likely, which has the efiect of defocusing the beam with consequent adverse tube performance. Such defocusing results in spreading of the electrons of the beam. In addition, the high orbital motion produces large centrifugal forces on the electrons which in the absence of compensating influences would remove the electrons from the tubular confines of the desired beam and thus, these electrons would be lost from the beam and tube ,efiiciency impaired.

Accordingly, it is an object of my invention to produce a thin tubular electron beam of high orbital energy extending from the region of a cathode to an annular interaction region of an electron tube.

It is another object of my invention to increase the efliciency of electron tubes utilizing tubular electron beams by utilizing a greater proportion of cathode emitted electrons in the beam.

It is another object of my invention to compensate for the centrifugal force acting upon electrons traveling in a helical path.

In accordance with my invention, a thin tubular electron beam having a high proportion of its kinetic energy in orbital movement is produced for interchange of energy in a tube interaction region by a magnetic radial field in space adjacent to an annular cathode through which electrons travel. For maintaining the beam thin and of substantially constant radius in the transition region between the annular cathode and an opening into the interaction region which is conguent with the cathode, a pair of circular inner electrodes is disposed within the beam path and a further pair of outer electrodes is disposed so as to surround the beam path. The electrodes extend axially from theregion of the cathode to the interaction region and for producing an electric field in the transition region which has a progressively increasing component directed radially inwardly from the region of the cathode to the interaction region, the inner electrode adjacent to the cathode is decreased in radius along an axial direction away from the cathode and the other electrodes are increased in radius along an axial direction away from the cathode. The particular contour of the electrodes is determinable by electrolytic tank procedures and are preferably stepped to facilitate construction. By the application of the same'potential, very slightly negative with respect to the cathode, to the electrodes adjacent to the cathode and the same, relatively high positive potential with respect to the cathode to the other electrodes, the space varying field with progressively increasing radially inward component across the beam path is produced whereby the electrons having progressively increased orbital speed and centrifugal force are maintained in a substantially constant radius orbit. The longitudinal component of the electric field is efiective to attract electrons to the interaction region.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which I regard as my invention, the invention will be better understood from the following description taken in connection with the accompanying drawing in which:

Figure 1 illustrates in partial elevational cross section, an electron structure for an electron tube apparatus,

Figure 2 illustrates schematically the space varying electric field for compensating for centrifugal force on electronsalong the electron beam path,

Figure 3 illustrates the tube shown in Figure 1, with a portion of the housing about the interaction region cut away to show the interaction structure and collector and Figure 4 is a view taken along figure 4-4 of Figure 3, illustrating a slow wave structure which may be used in the tube embodying the invention.

Referring now to Figure l of the drawings, 10 represents generally an entire electron tube structure embodying an electron gun according to my invention. The tube includes a hermetically sealed housing 12 of nonmagnetizable metal for enclosing the operable tube components. Within the housing 12 and near one end thereof is an electron gun according to my invention and represented generally at 14. Within another portion of the housing is an interaction region 15 having an outer conduetor 16 and a slow wave structure 17 in which region a beam of electrons from the gun interacts with the structure in this region for an interchange of energy from the tubes direct potential sources to the high frequency field. Between the interaction region and the gun structure is an inner conductor 18 concentric with conductor 16 and of substantially the same radius as structure 17.

For producing a tubular electron beam as indicated by the dashed lines at 19 in Figure 1, the gun comprises an annular cathode 21 having a surface 22 coated with a suitable thermionic emission enhancing material. The cathode is responsive to appropriate heating to emit the electrons into space adjacent to the cathode and under the influence of a component of an electric field produced by electrodes shown at 23, 24, 25 and 26, the electrons are attracted along a path substantially perpendicular to the cathode surface 24 to be collected by a collector 27 at the end of the tube remote from gun 14. However, a magnetic field in space adjacent to the surface 24 and radially directed is produced by electromagnet windings 28 and 29 differentially energized and because of the Lorentz effect on the electrons, an orbital motion is imparted to the electrons as they move along a longitudinal path. The electrons, under the influence of the longitudinal component of electric field, are accelerated whereby the Lorentz effect progressively increases along their path to increase the orbital motion and the effect of the magnetic field is aggregative to still further increase the orbital motion.

The intensity of the magnetic field may vary in accordance with the proportion of the total kinetic energy of the beam that is in the form of orbital motion. In tubes wherein it is desirable to achieve interaction between a beam and slow wave structure with the energy predominantly in longitudinal flow, the magnetic field may be relatively low as compared with the field necessary to produce a beam havinga high proportion of the kinetic energy of the beam in orbital motion and in which case the beam interacts with a slow wave structure in the angular or direction. .A typical interaction structure for 'beam interaction in the 0 direction may be as shown more clearly in Figures 3 and 4 of the drawings wherein it comprises a plurality of four elongated members 17A,

17B, 17C and 17D having arcuate cross sections and being disposed in generally circular form and spaced apart along their lengths.

The centrifugal forces acting on the electrons due to their orbital motions is etiective to tend to increase their radii of orbit. However, in accordance with an important feature of my invention, the effect of the centrifugal forces acting on the electrons are overcome by a component of the electric field produced by the mentioned electrodes and acting along the electron path and directed radially inwardly. The inwardly directed component of field is of progressively increasing magnitude along the beam path and is of a magnitude to effectively overcome and compensate for the progressively increasing centrifugal force acting on electrons.

The electrodes 23 and 24 are axially spaced along the interior of the beam path and have a maximum radius less than cathode 21 and electrodes 25 and 26 are hollow and surround the beam path at radii greater than the beam path.

The inner electrodes 23 and 24, together with cathode 21 are mounted in axial, stacked and insulated relationship along a central, inner portion of the housing. To

. 4 I facilitate such a mounting, cathode 21 is supported by a conductive, centrally apertured disc 30 which in turn is connected to an annular, conductive member 32. The conductive member 32 is spaced on one side from an end wall 34 of housing 12, by a centrally apertured washer 36 of insulating material and on the other side from electrode 23 by a centrally apertured washer 38 of insulating material. Electrode 26 is spaced from electrode 28 by a centrally apertured washer 400i insulating material. The member 32, the electrodes 23 and 24 and washem 36, 38 and 40 are maintained in tightly stacked juxtaposition on a plurality of axially mounted metal studs extending through aligned apertures in these elements, one of which is shown at 42. Each stud is fixedly secured at one end to end wall 34 and extends through the aligned openings in the stacked elements. For insulating the member 32 and electrodes 23 and 24 from the stud 42, the openings in these elements are enlarged to accommodate respective insulators 44, 46 and 48 which are also apertured to accommodate the stud. At the end of the stack remote from wall 34, the stud 42 is insulated from electrode 24 by a centrally apertured insulating washer 50 having an opening therein to accommodate stud 42.

- To tighten the stack, stud 42 is threaded at its end remote from wall 34 to threadedly engage a nut 52 having a washer 54 interposed between it and washer 50. The nut may be threaded along stud 42 to tighten the array. As hereinabove noted, other studs, similarly mounted and interrelated with the stacked elements are provided but not shown in the drawings to facilitate clarity.

Support for one end of inner conductor 18 of the cylindrical capacitor is provided by washer 50 having a reduced portion 56 extending within the conductor 18 and a shoulder 58 against which the end of conductor 18 abuts.

Emission from the cathode is facilitated by an electrical resistance heater element 60 disposed in an annular channel 61 formed adjacent to cathode 21 remote from surface 22 and conductive leads 62 and 64 connected to respective ends of the heater element enable supply of electrical power to the heater.

For applying biasing potentials to cathode 21, electrodes 23 and 24 and conductor 18, conductive leads 66, 68, 70 and 72 extend through an insulator 74 disposed in a central opening in wall 34 and are conductively con nected, respectively, to member 32, electrodes 23 and 24 and conductor 18.

A mounting similar to that described for supporting electrodes 23 and 24 supports electrodes 25, 26 and 33 within housing 12 at a radius greater than cathode 21. Metal studs such as that shown at 76 are secured at one end to wall 34 and extend axially therefrom through aligned openings in the electrodes 25 and 26 and through a conductive annular supporting member 77. These respective electrodes and member are insulated from stud 76 by respective insulators 78, 80 and 82 disposed in these openings and the insulators have openings to accommodate the stud 76. For insulating electrode 25 from housing 12, an annular washer 84 of insulating material is disposed between wall 34 and electrode 25 and is also provided with openings to accommodate the studs such as 76. Electrode 25 and member 77 are spaced apart and insulated by an annular washer 86 of insulating material disposed therebetween and also having an opening to accommodate the stud 76. Member 77 and electrode 26 are spaced apart and insulated from each other by a washer 88 of insulating material and an annular insulator 90 apertured to accommodate stud 76 is mounted at the end of the outer stacked array. The array is tightly secured by a nut 92 threadedly engaging the stud end and a washer 94 disposed between the nut and washer 90. The washer 90 is cut away at 96 at an inner portion thereof to receive and support conductor 16 and the end of the conductor 16 abuts a shoulder 98 to limit its axial position.

For applying biasing potentials -to the electrodes 25, 26

and 33, and conductor 16, conductive connections to these electrodes and conductors are provided by conduc tive leads extending through the housing 12 in insulated relationship therewith. Thus, lines 100, 102, 104 and 106 extend through the housing and pass through respective insulators 108, 110, 112 and 114 and connect, respectively, with conductor 16, member 77 and electrodes 25, 26 and 33.

For establishing the steady magnetic flux in the region adjacent to the cathode surface 22 and parallel therewith, the wire coils 27 and 23 are concentrically disposed on respective, concentric, tubular magnetizable yokes 118 and 120 which are in abutment with wall 34. The yokes engage respective outer and inner radial surfaces 122 and 124 of an annular recess 126 formed adjacent the exterior side of wall 34 by a pair of tubular flanges 128 and 130. A set screw 132 threadedly by engaging a radial bore in flange 130 is tightly engageable with yoke 118 for retaining the same in position and at the ends of the yokes remote from wall 34, they are formed integral by an end wall 132. Leads 66, 68, 70 and 72 extend through insulators disposed in apertures in this wall for external connections.

In the gun of Figure 1, the heater 60 may be electrically energized sufiiciently to raise cathode 21 to a temperature of copious thermionic emission and the electrons under the influence of the electric field produced by the potentials applied to the electrodes 23, 24, 25, 26 and 33 tend to travel longitudinally along the dotted lines 19. However, the fringing of magnetic field produced by coils 27 and 28 having radially directed lines adjacent to the surface 22 of cathode 21, is effective under the Lorentz eflfect, to impart an orbital motion to the electrons whereby they travel both longitudinally and orbitally or in other words, in a helical path along the gun from the cathode. Each electron in its travel progressively traverses the magnetic field and is therefore continuously accelerated angularly. In addition, the longitudinally accelerating influence of the electric field further accelerates the orbital motionthereof. Thus, the centrifugal force acting upon the electrons in their travel away from the cathode, progressively increases.

According to a feature of my invention, the electrodes 23 and 24 on the one hand and electrodes 25 and 26 on the other hand, are contoured and spaced with respect to each other to produce a quiescent but space varying elec tric field in the gun, in response to appropriate biasing potentials applied to these electrodes and which field is directed to produce forcecs on the electrons to effectively counter the progressively increasing centrifugal force acting on the electrons. The effect on the electrons is to constrain the same to motion within the confines of a desired tubular volume. This is illustrated in Figure 2 of the drawings wherein cathode21 is reproduced and the dashed lines 19 again represent the electron beam. The field produced by the electrodes of my invention is represented by the arrows which progressively have an increased component in the radially inward direction with increased distance from the cathode.

For determining the shape and spacing of the several electrodes of the gun according to my invention which are required for producing an electric field efiective to counter the centrifugal forces acting on the electrons, their orbital velocity at constant radius may be mathematically calculated from which the centrifugal forces and necessary electric field intensity may also be mathematically calculated at all points along the beam. The equation of this potential expressed in terms of magnetic field intensity, velocity, distance from cathode is as follows:

iew-ti wherein '=field potential in volts at any point along the beam 1 =e/m (electron charge to mass ratio) in c Q=electron angular momentum per unit mass (P m) in kilogram (meters) per second r =mean radius of beam in meters r='radial position of an electron able by the solution of the following simultaneous differential equations.

n=selected value in accordance with magnetic field structure.

w is calculated from the gun perveance and fraction of beam energy in rotation In accordance with known electrolytic tank procedures, with the above-mentioned equation the shape and disposition-of the electrodes required "to produce the required compensating field may readily be determined by trial and error techniques to a high degree of accuracy. Thus, as shown in Figure 1, such electrodes may include the inner electrodes 23 and 24 which are formed to have changes in radius along their axial dimension and. wherein electrode 23 is reduced in radius along the axial dimension and electrode 24 is increased in radius along the axial dimension. Also, electrode 25 is progressively increased in radius in an axial direction away from cathode 21 and electrode 26 is similarly increased in radius in a direction away from the cathode. Electrode 33 may be simply of a single step and is useful in forming the proper field in the space between the gun and the interaction region. The electrodes 23, 24, 25 and .26 are each shown as having stepped changes in radius and for convenience in manufacture these electrodes may he so constructed. However, in accordance with'my inventlon these electrodes may have gradual or smooth changes in cross-section rather than stepped changes and the same desired field effects maybe achieved.

The electrodes are electrically biased to produce an electric field as indicated in Figure 2 of the drawings and as an example, to achieve such a field, electrodes 24 and 26 may be of equal potential V,,, electrodes 23 and 25 may be of the same, slightly negative potential of approximately .01 V and electrode 33 may be at a potential intermediate to these potentials. In response to such electrode configuration and biasing, a field as shown in Figure 2, is produced wherein the radial, inward component is progressively larger along the axial dimension of the gun to compensate for or counter the progressively increased centrifugal force on the electrons. It is, of course, to be understood that in accordance with my invention, the particular number, position,-shape and potential bias applied to the electrodes may be'varied according to the electrolytic tank techniques herein mentioned and that it is essential only to produce the field as indicated in Figure 2 which counters the radial centrifugal forces on the electrons in the beam along its length in the gun.

As the beam enters the interaction space between conductors 16 and 18, the magnetic field is of virtually no effect in imparting orbital acceleration to the electrons and thus, the constant potential difference between these concentric conductors is effective in maintaining the beam radius within acceptable limits. In accordance with the above-mentioned example, the potential, of conductor 18 may be somewhat greater than V,, and conductor 16 may be substantially at cathode potential, or in other words,

zero.

While the present invention has been described by reference .to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. 1, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing dis closure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An apparatus comprising an annular thermionica-lly emissive cathode surface for producing an electron beam, means producing aradially directed magnetic field in space adjacent to said surface for imparting orbital motion to electrons emitted from said surface and a plurality of electrodes on each side of an axial projection of said cathode and being electrically biased to apply forces to electrons in said beam overcoming the centrifugal forces acting on said electrons resulting from said orbital motion.

2. An apparatus comprising means producing a tubular beam of electrons, means imparting simultaneous longitudinal and orbital motion to said electrons, means maintaining said electrons at a substantially constant radius including a plurality of electrodes on each side of said beam and beingelectrically biased to produce an electric field through said beam effective to counter the centrifugal force on said electrons at all points along said beam.

3. An apparatus comprising means producing a tubular beam of electrons along a path, means imparting simultaneous longitudinal and orbital motion to said electrons, means maintaining said electrons at a substantial ly constant radius in said beam path including electrodes within said beam path and electrodes surrounding said beam path, said electrodes being electrically biased with respect to said cathode and with respect to each other to produce an electric field across said beam path having a component accelerating said electrons longitudinally along said beam path and a component countering the centrifugal force on the electrons of said beam.

4. An apparatus comprising means producing a tubular beam of electrons along a path and having a circular cross section and a component of orbital velocity, means including a plurality of electrodes surrounding said beam path, said electrodes being shaped and disposed with respect to each other to produce an electric field across said beam path having a space varying, radially inward component in response to potentials applied thereto whereby said radial component is effective to counter the centrifugal forces acting on said electrons.

5. An apparatus comprising an annular electron source and a magnetic field in a radial direction adjacent to said source, means for producing a tubular beam of electrons of circular cross section along a predetermined path and including a first pair of circular electrodes of radius smaller than said source and being disposed within said beam path, the first of said electrodes having portions of decreased radius in a direction away from said source and the second of said electrodes having portions of increased radius in'a direction away from said source, a plurality of circular electrodes external to said beam,

each of said pluralities of electrodes having portions of increased radius in a direction away from said source whereby said electrodes are responsive to bias potentials applied thereto to produce a field across said beam having a component inserted radially inwardly and increasing progressively in magnitude in a direction away from said source to balance the centrifugal forces acting on said electrons in said beam.

6. An apparatus according to claim 5 wherein said second of said pair of electrodes is on the side of said first of said pair of electrodes remote from said source.

7. An apparatus comprising an annular electron source and a magnetic field in a radial direction adjacent to said source, means for producing a tubular beam of electrons of circular cross section including a first pair of circular electrodes of radius less than said source and spaced axially therefrom in coaxial relationship therewith, the first of said electrodes having portions of decreased radius in a direction away from said source and the second of said electrodes having portions'of increased radius in a direction away from said source, a second pair of circular. electrodes of radius greater than said source spaced axially from said source and being coaxial therewith, each of said second pair of electrodes having an increased radius in an axial direction away from said source, means applying a predetermined potential to one of said first pair and one of said second pair of electrodes and means applying a fraction of said potential to the other elec'- trodes of said first pair and said second pair for producing an electric field potential along said beam having a progressively increasing radially inward component in accordance with the equation -ieieer a A =angular component of magnetic field potential in wherein z=axial distance of electron from origin in meters t=time in seconds.

8. An apparatus comprising an annular electron source and a magnetic field in a radial direction adjacent to said source, means for producing a tubular beam of electrons of circular cross section including a pair of inner electrodes each of radius less than' said source and a pair of outer electrodes each of radius greater than said beam, said electrodes being coaxial with said source and having portions spaced axially therefrom, one of said inner electrodes being of decreased inner radius in a direction away. from said source and the other electrodes being of in creased inner radius in a direction away from said source, means applying a predetermined potential bias with respect to said source to an inner and outer electrode and means for applying a potential bias equal to a fraction of said predetermined potential bias to the other of said electrodes to produce a field between said outer and inner electrodes having a progressively increasing component directed radially inwardly in space away from said source.

9. An apparatus comprising means for producing a tubular beam of electrons along a predetermined path, means for establishing a magnetic field across said path and in a radial direction with respect to said tubular beam for imparting an orbital motion to said beam, means for maintaining the electrons in said beam at a substantially constant radius along the beam and including a plurality of electrodes within and externally of said beam, said electrodes being electrically biased with respect to said cathode and with respect to each other to produce an electric field having a component for accelerating electrons along said path and a radially, inwardly directed component, progressively increasing with distance away from said cathode and progressively imposing increased electric field forces on the electrons of the beam in a direction away from said cathode.

10. An apparatus comprising means for producing an 1O path for imparting an orbital motion to the electrons of said beam, means for maintaining the electrons of said beam at a substantially constant radius and including means disposed along said beam and on opposite sides thereof for producing an electric field having a component along said path for accelerating the electrons of said beam along said path and an inwardly directed radial component progressively increasing along said path for countering the efifects of increased centrifugal force acting on said electrons along the path.

References Cited in the file of this patent UNITED STATES PATENTS 2,554,134 Wagener May 22, 1951 2,608,668 Hines Aug. 26, 1952 2,652,512 Hollenberg Sept. 15, 1953 2,752,523 Goodall June 26, 1956 2,753,484 Bryant July 3, 1956 2,791,711 Harris May 7, 1957 2,843,793 Ashkin July 15, 1958 

